Color reversal photographic elements typically use a silver halide emulsion, the grains of which form a surface latent image upon exposure to light. The element, following exposure, is processed by first developing in a black and white developer. This develops any exposed silver halide grains. Treatment with a black and white developer is followed by a fogging step which renders any unexposed silver halide developable. Subsequent treatment with a color developer develops the unexposed grains and thereby produces oxidized color developer which then reacts with a dye forming compound (usually a color coupler) to produce dye in unexposed regions of the element. Currently, the particular process which has become a standard for processing reversal films, is by Kodak Process E-6 or substantially equivalent processes made available by other manufacturers.
It will be noted that color reversal elements generally have higher contrasts and shorter exposure latitudes than color negative film. Reversal elements also have a gamma generally between 1.8 and 2.0, and this is much higher than for negative materials. Moreover, such reversal elements do not have masking couplers, and this further differentiates reversal from negative working films.
Color reversal photographic materials though, are often used under insufficient light conditions. In such situations, to obtain useful images in the underexposed areas of the films, the films are often developed for longer than standard times. This extended processing is often referred to as "push processing". Push processing is generally achieved by extending the development time of the first developer (B&W). Another reason for push processing is to modify the response of the element in the low exposure regions. That is, a photographer may want to have higher toe contrast than the normal film/process provides (that is, higher contrast in the higher exposure region of the density versus logE curve of a reversal element). The control of toe contrast can also be achieved by extending first development time (that is, by push processing). Usually, push processing is measured in terms of "stops pushed". Thus, in the case of Kodak process E-6, the normal first (black and white) development time is 6 minutes. However, for a "1 stop push", that time is increased to 8 minutes. For a 2 or 3 stop push, the first development time would be increased to 11 or 13 minutes, respectively.
Push processing of conventional color reversal light sensitive materials can produce several undesirable defects. These include:
1. Sufficient speed increases may not be attained unless first development time is extremely prolonged.
2) Undesirable changes in curve shape may result.
3) The highest density (Dmax, low exposure area) that can be attained may be lowered to a degree where image quality is no longer acceptable.
4) Color mismatch may result from mismatch of the development rates in the red, green, and blue light-sensitive layers of multilayer films.
U.S. Pat. No. 2,996,382 describes a technique of enhanced speed and contrast of iodide containing emulsions by incorporating a combination of unfogged surface latent image silver halide grains and fogged internal latent image silver halide grains in an emulsion layer. U.S. Pat. No. 3,178,282 extends the technique of U.S. Pat. No. 2,996,382 to non-iodide containing emulsions by using solvent containing developers.
U.S. Pat. No. 4,626,498 describes the use of a combination of unfogged surface latent image silver halide and internally fogged silver halide grains (IF) in push processing of color reversal materials. A disadvantage of this technique is that the image density decreased upon push processing (Dmax loss) which accompanied the speed and contrast increase. U.S. Pat. No. 4,886,738 describes a technique aimed to maintain the sensitivity and contrast advantages in push processing without the decrease of maximum density, by using a combination including inhibitors and surface or internally fogged silver halide grains.
U.S. Pat. No. 4,839,268 discloses a color reversal element which uses an emulsion of grains which form a latent image "mainly inside the grain" as stated in the patent. The object of the patent is to provide color reversal materials having good sharpness and a high contrast when first development time is prolonged or is performed with increased temperature.
Other techniques which have been used to enhance push processing are described, for example, in U.S. Pat. No. 5,041,367, which discloses the use of Lanothane and 4-carboxymethyl-4-thiozoline-2-thione to enhance speed gains. U.S. Pat. No. 4,444,865 discloses the use of a combination of internally sensitized core shell type emulsions with other internal latent image forming core-shell emulsions or with internally fogged emulsions, to enhance the covering power of an image in direct positive elements. A combination of surface fogged emulsions with surface latent imaging emulsions is disclosed in U.S. Pat. No. 4,082,553 to improve interimage effects.
Thus it is desirable to provide a method that allows for push processing to obtain a good speed increase in a color reversal film, or to control the curve shape, with relatively low loss of maximum density (D.sub.max). It is also desirable that such means can allow independent control of the extent of push processing in the individual emulsion layers.